add cross-fork development support files

[bill-auger]

• added cross-fork config skeletons to config/cross-fork-dev/
• added cross-fork merged Gemfile to config/cross-fork-dev/
• added cross-fork-dev-README.md to config/cross-fork-dev/
• add db/seeds.rb to .gitignore

[bill-auger]

the first four commits are in common with pr #105